1. Field of the Disclosure
Embodiments disclosed herein relate generally to downhole screens. More particularly, embodiments disclosed herein relate to apparatus and methods for a high pressure screen test fixture.
2. Background Art
During drilling of a wellbore, the pressure balance between the circulating drilling fluids and that of the formation being drilled may be maintained in an underbalanced or an overbalanced mode. Underbalanced drilling is a method of drilling a desired subterranean formation where the hydrostatic pressure exerted by a column of drilling fluid in the drill string is less than the natural pressure (pore pressure) inherent in the subterranean formation being drilled. Underbalanced drilling may prevent damage to the desired subterranean formation and in particular low pressure formations. Typically, the pressure differential is set to provide a margin above the pressure at which wellbore collapse might occur. The introduction of sufficient air, nitrogen or other gases to the drilling fluids may reduce the density of the commingled fluids and effectively decrease hydrostatic pressure. Other low density fluids, such as emulsions, foams and mists, may be used as a drilling fluid to achieve an underbalanced condition.
In overbalanced drilling, fluid in an annulus of a well is used to exert a pressure that is greater than the formation pressure. The mud weight, or density, may be calculated to give the appropriate pressure gradient across the exposed formation to provide the optimum fluid migration rate into the least stable horizon of the exposed formation. Thus, the pressure that is exerted by the annular fluid prevents formation fluids from exiting the well and may provide support for the wellbore. A drawback to this technique is that particulates added to increase the weight of the fluid (and, thus, increase its downhole pressure), as well as other particulates, emulsified fluids, and surfactants, may be pushed into the formation and damage the formation. The well may also need to be tested after overbalanced drilling to check for formation damage.
After the desired borehole in the hydrocarbon reservoir is drilled, production tubulars and/or screens may be run to the bottom of the borehole and placed against the desired formations for hydrocarbon production. When the hydrocarbon-bearing formations consist of poorly cemented sands, sand control methods or devices are used to prevent sand particles in the formation from entering and plugging the production screens and tubulars in order extend the life of the well. One typical sand control method includes filling an annular space between the wellbore and the production screens with specially sized sand, which is usually larger than the formation sand and commonly known as gravel pack sand. The process of placing the sized sand behind the production screen is known as a gravel pack operation.
Gravel packing involves the complete placement of selected gravel across the production interval to prevent production of formation fines or sands. Any gap or interruption in the pack coverage may allow undesirable sand to enter the producing system. Referring to FIG. 1, a wellbore 100 with a gravel-pack packer 102 is shown. Gravel-pack packer 102 may be set in casing 104 with gravel-pack screen 106 being placed in a perforated zone 108. Gravel 110 may be placed in casing 104 and may flow into perforations 108.
In addition to the appropriate use of underbalanced or overbalanced drilling, another way to protect the formation is by forming a filter cake on the surface of the wellbore, or on the downhole screens described above. A filter cake is a tough, dense, practically insoluble residue composed of either soluble or insoluble materials that reduces the permeability of the formation and which is formed when particles or emulsified fluids suspended in a drilling fluid coat or plug the pores in the subterranean formation while drilling overbalanced. Filter cakes may be formed a number of ways known in the art, including the use of both clay and non-clay based drilling fluids. Sealing off producing formations using a filter cake may also be desired in order to prevent fluid loss and possible damage to the formation. Filter cakes can prevent loss of drilling fluids to the formation by substantially preventing fluids from passing between the wellbore and the formation. Formation of a filter cake may also be desired prior to completion or workover of a well. In this case, a filter cake may be formed on the inside of the production, or gravel pack, screen for the purpose of limiting fluid loss to the reservoir through the screen.
Advances in oilfield technology have led to drilling of deeper wells in increased water depths. As a result, drilling and workover operations often encounter declining formation pore pressure as existing producing wells are depleted. A higher overbalance condition is therefore required for successful workover operations. Accordingly, there exists a need for screens suitable for use at increased working pressures. Likewise, there exists a need for apparatus and methods to test screens and drilling/workover fluids at these increased working pressures.